Among the various types of batteries available at present, lithium ion batteries are attracting attention from the viewpoint of their high energy density. Among these batteries, all-solid-state batteries, in which the electrolytic solution has been replaced with a solid electrolyte, are attracting particular attention. This is because, differing from secondary batteries using an electrolytic solution, since all-solid-state batteries do not use an electrolytic solution, there is no degradation of the electrolytic solution caused by overcharging and these batteries have high cycling characteristics and high energy density.
Olivine-type positive electrode active materials are known to be used for the positive electrode active materials used in lithium ion batteries. Olivine-type positive electrode active materials have a more stable structure and higher cycling characteristics in comparison with other positive electrode active materials. Consequently, research has recently been conducted on all-solid-state batteries using olivine-type positive electrode active materials.
Patent Document 1 discloses a positive electrode active material, obtained by coating a type of olivine-type positive electrode active material in the form of LiFePO4 with Li3PO4, and a method for producing that positive electrode active material.
Patent Document 2 discloses an all-solid-state battery having a polyanionic positive electrode active material such as LiFePO4 and a sulfide solid electrolyte, wherein a sulfide layer containing iron ions is formed at the interface between the positive electrode active material and sulfide solid electrolyte by mixing the LiFePO4 with the sulfide solid electrolyte followed by firing in an attempt to optimize interface bonding.